1. Field of Use
While the invention is not limited to any particular field it has specific utility in the production of printing plates, pattern plates and other marking and shaping items.
2. Description of Prior Art
In the newspaper industry, for many years printing plates have been made of lead formed against a matrix of wood pulp, clay and water.
Recently, to expedite printing plate duplication, plastic printing plates have been tried on a limited scale in place of the lead plates. In one process, a magnesium metal master or pattern plate is prepared by a photographic and etching process. A negative (or matrix) is then formed by pressing a plastic against the metal to form a reproduction (negative) of the master plate. The plastic matrix is then used to produce a number of "positive" copies for the actual printing function when multiple press operation is employed. The copies ("positives") are formed by molding or pressing thermoplastics or rubber elastomers against the "negative" matrix.
In another process, instead of the metal "master" an organic plastic "master" or pattern plate is made using a liquid polyester polymer which is cured by transmitting ultra-violet light through a photographic negative or transparency of the printed format. In the areas struck by the ultra-violet light, the resin is hardened. In the areas "screened" from the ultra-violet light, the resin remains liquid and is washed away by an aqueous developing solution.
The polyester master or "positive" is then used as described above as a substitute for the metal master to make a "negative" matrix which in turn can be used to produce multiple "positive" copies. The copies are then used as the actual printing plates. (A process using the polyester concept is sold by Hercules Incorporated, Wilmington, Del. under the trademark "Merigraph".)
However, the above prior processes have inherent deficiences as the matrix plates (negative plates) have been susceptible to taking a compression-set on repeated uses. As a result, the printing plate copies or duplicates made from the negative matrix have letters of different heights. The printed copy is thus blurry, smudged, and hard to read. Certain areas might have too much ink and others too little.
One conventional matrix concept uses a non-dense core of an asbestos-cellulose composition which has a phenolic resin coating. However, when a moldable material such as polypropylene is molded against the matrix, the core gradually flows and compresses in thickness providing uneven print characters. Moreover, the phenolic resin of the matrix is susceptible to flex-cracking and peeling from the core material.
Other problems have been inherent with the prior phenolic-type matrix plates. When formed against the organic polyester "masters", the pressures required to form the phenolic "negative" matrix have caused the print or characters on the organic polyester to collapse. The phenolics also have a tendency to stick and bond to the polyester. In addition, the phenolic matrix plates cannot be bent easily to the curved surface of a mold platen (having substantially the same diameter as a printing press cylinder) without cracking.